palaeogene forearc sedimentary fill on lemnos...

A. MARAVELIS & A. ZELILIDIS

415

Porosity-Permeability and Textural Parameters of the

Palaeogene Forearc Sedimentary Fill on Lemnos Island,

NE Greece

ANGELOS MARAVELIS & AVRAAM ZELILIDIS

University of Patras, Department of Geology, Laboratory of Sedimentology, 26504 Patras, Greece

(E-mail: [email protected])

Received 04 October 2010; revised typescript received 10 January 2011; accepted 08 February 2011

Abstract: Th e Palaeogene forearc sedimentary fi ll on Lemnos Island, NE Greece, was examined to determine reservoir

characteristics and textural parameters. During this time interval the studied area was the site of accumulation of

submarine fans that underlie shelf deposits, with tectonic activity responsible for the shallowing upward trend.

Turbidites were deposited in both inner (slope fan facies) and outer parts (basin fl oor fan facies) of the submarine fan

system and consist of alternating sandstone and mudstone beds. Sandstones occur in both complete and incomplete

Bouma sequences while shelf deposits have been interpreted as storm-surge deposits on the deeper parts of shelves. Th e

‘Mercury Porosimetry Technique’ was carried out on 20 sandstone samples, while 30 sandstone samples were examined

under a polarizing microscope for grain-size analysis. Th is porosity-permeability study suggests that these rocks can be

considered as both oil and gas reservoirs. ‘Slope’ fan facies generally reveal the most effi cient values, making them the

most promising sub-environment for further hydrocarbon research. Most samples display two pore-size distributions

suggesting major textural heterogeneity. Textural parameter analysis reveals that sorting was of great importance during

sedimentation. Rocks are generally well to very well-sorted while samples with moderate sorting are also present. Th is

fact can be both attributed to the restricted grain-size range and their possible great distance from the source area. Th is

generally well-sorted sequence argues well for further hydrocarbon research in the Northeast Aegean Sea, since the

higher the sorting the higher the porosity. Selected samples are generally very fi ne to fi ne grained, whereas medium-

grained sandstones are extremely rare and mostly seen on ‘slope’ fan facies. Th e fi nest grained sandstones are the best

sorted.

Key Words: porosity, permeability, textural parameters, grain-size, Lemnos Island, NE Greece

Lemnos Adası Paleojen Hendek-önü Tortul Dolgunun Gözeneklilik-Geçirimlilik

ve Dokusal Öğeleri, KD Yunanistan

Özet: KD Yunanistan’da Lemnos adası Paleojen hendek-önü tortul dolgusu, hazne özelliklerinin ve dokusal öğelerini

belirlemek üzere çalışılmıştır. Bu zaman aralığında çalışma sahası, tektonik etkinlik denetiminde üste doğru sığlaşan

ve şelf tortulları ile üzerlenen denizaltı yelpazelerinin birikim alanı konumundaydı. Kumtaşı ve çamurtaşı tabakaları

ile ardışımından kurulu türbiditler denizaltı yelpazesinin iç (yamaç yelpaze fasiyesi) ve dış (havza tabanı yelpaze

fasiyesi) kesimlerinde depolanıyordu. Şelf tortulları, şelfi n daha derin kesimlerindeki fırtına-kabarma tortulları olarak

yorumlanırken kumtaşları ise tam veya eksikli Bouma istifi şeklindedir. 30 kumtaşı örneğinin tane boyu polarize

mikroskopta belirlenmiş, 20 kumtaşı örneğinde ise Mercury gözenekliliği kullanılmıştır. Gözeneklilik-geçirimlilik

çalışması bu tür kayaların petrol ve gaz haznesi olabileceğini göstermiştir. En verimli değerleri sunan yamaç yelpaze

fasiyesi, ilerdeki hidrokarbon aramaları için en ümitli alt-ortamdır. Çoğu örnekler ana dokusal heterojenlik sunan iki

boşluk-boyutu dağılımını sunar. Dokusal çalışmalar boylanmanın depolanma sırasında kazanılan önemli bir unsur

olduğunu göstermiştir. Kayalar, orta derece boylanma ile birlikte genellikle iyi-çok iyi boylanmışlardır. Bu durum kısıtlı

tane boyu aralığı ile birlikte olasılıkla kaynak alandan çok uzak mesafede oluşu gösterebilir. Genellikle iyi boylanmış

istifl er daha yüksek boylanma ve gözenekliliklerinden ötürü KD Ege Denizi’nde ilerde yapılacak hidrokarbon

aramalarını gündeme getirir. Seçilmiş örnekler genellikle çok ince-ince taneli iken, orta-taneli kumtaşları oldukça

nadirdir ve çoğunlukla yamaç yelpaze fasiyesinde görülürler. Öte yandan en ince taneli kumtaşları en iyi boylanmıştır.

Anahtar Sözcükler: gözeneklilik, geçirimlilik, dokusal öğeler, tane boyu, Lemnos Adası, KD Yunanistan

Turkish Journal of Earth Sciences (Turkish J. Earth Sci.), Vol. 21, 2012, pp. 415–438. Copyright ©TÜBİTAK

doi:10.3906/yer-1009-29 First published online 08 February 2011

PALAEOGENE FOREARC SEDIMENTARY FILL OF LEMNOS ISLAND

416

Introduction

Deep-water turbidite reservoirs are important

exploration targets worldwide (Weimer & Link

1991). Most published work on such play targets is

primarily focused on their spatial confi gurations

and characteristics, sedimentary processes and

distribution patterns, as well as slope and base

of-slope fan systems and their eff ects on the

distribution, quality, and reservoir architecture of

submarine turbidite reservoirs. Th ese reservoirs

oft en have complex architectures and lithological

variations. Th ere is a wealth of literature on reservoir

characterization, diagenetic cements, and reservoir

heterogeneities (Prosser et al. 1995; Watson et al.

1995; Dutton et al. 2003).

Predicting subsurface porosity and permeability

is a key challenge for hydrocarbon exploration and

development when there is little subsurface data

available. Samples from outcrops may provide an

important source of data for the study of correlative

reservoirs and provide the exploration geoscientists

with the opportunity of observing sedimentary

structures, lateral facies changes, and three-

dimensional spatial relationships of correlative

subsurface rocks. Outcrop-based samples also help

geologists understand the burial history and the role

of diff erent diagenetic modifi cations on reservoir

properties, which lead to prediction of porosity and

permeability of subsurface reservoirs (Tobin 1997).

Th e exploration of turbidite sandstones is

complicated since the quality of sandstone reservoirs

in continental deposits is known to be aff ected by

various geological processes, such as tectonic setting,

depositional environment, mineral composition

and basin fl uid fl ow (Surdam et al. 1989; Gier

2000; Ketzer et al. 2002; Sachsenhofer et al. 2006).

Although turbidite plays of the NE Aegean Sea (e.g.,

Lemnos) could serve as possible loci for hydrocarbon

accumulation (Maravelis & Zelilidis 2010a); little is

known about their reservoir quality and its controls

on these Palaeogene sandstones. Th e objective of

this study is to provide original sedimentological,

porosity-permeability and textural parameter data to

the study of these clastic sediments in order to defi ne

reservoir porosity.

Geological Setting

Th e study area lies in NE Aegean Sea, Greece. Th e plate confi guration of the Aegean region (Figure 1) consists of the Aegean Plate to the south separated by a strike-slip boundary (McKenzie 1970; Papazachos et al. 1998) from the Eurasian Plate to the north, which encompasses the north Aegean, Rhodope and adjacent areas. Th e Aegean Plate is overriding the African Plate, accommodated by northeastward dipping subduction in the Hellenic Trench. Th e strike-slip boundary between the Aegean and the Eurasian plates (the north Aegean transform zone) consists of two major strike-slip faults, which are extensions of the North Anatolian Fault. Convergence between the Eurasian and African plates has played a key role in controlling magmatism in the Balkan Peninsula since the Late Cretaceous period. During this time, collision resulted in the formation of several sub-parallel southward migrating magmatic belts with the youngest one being the present-day Aegean Arc (Fyticas et al. 1984).

During the late Eocene–early Oligocene, magmatic activity, caused by the subduction of the African Plate beneath the Eurasian Plate, occurred in the Macedonian-Rhodope-North Aegean region (Harkovska et al. 1989; Marchev & Shanov 1991). Th e magmatic belt extends to the NW into Skopje and Serbia, crossing the Vardar Zone (Bonchev 1980; Cvetkovic et al. 1995) and continues to the SE in the Th racian Basin and Western Anatolia (Yılmaz & Polat 1998; Aldanmaz et al. 2000).

A subduction mechanism has been proposed to explain late Cretaceous magmatism in the Rhodope Zone (Dabovski 1991). High-precision U-Pb zircon and rutile age dating in the Central Rhodope area indicates a southward shift of this magmatism from 92 to 78 Ma (Peytcheva et al. 2002). Th e progressive southward migration of magmatic activity in the Aegean region (Fyticas et al. 1984) that commenced in the Rhodope in the Late Eocene (Yanev et al. 1998), has been confi rmed by seismic tomography (Spakman et al. 1988), implying that present day north-vergent subduction in the Aegean region started at least 40 Ma ago.

It is generally believed that extension in the

Greek part of the Rhodope Zone did not start


417

EASTERN MEDITERRANEAN

TURKEY

GREECE

ITALY

SK

BULGARIA

AL

BA

NIA

SERBIA

CYPRUS

RZ

VZ

TB-WA

MACEDONIA

DE

AD

SE

AF

AU

LT

ZO

NE

ARABIAN PLATE

10 mm

/yr

ANATOLIAN

PLATE

BLACK SEA

AFRICAN

PLATE

10mm/yr

IONIAN SEA

ANDRIATIC

PLATE

EURASIAN

PLATE

AEGEAN

PLATE

45 mm/yr

SEA OF

MARMARA

DARDANELLES

NORTH ANATOLIAN

FAULT

EAST ANATOLIAN

FAULT

25 mm/yr

LEMNOS

STRIKE SLIP E-W EXTENSION

N-S EXTENSION

SUBDUCTION

CONTINENTAL

COLLISION

HELLENIC

TRENCH

NE AEGEAN

TRANSFORM ZONE

LEGEND

N

35°33°31°29°27°25°23°21°19°17°15°13° 37°

39°

41°

43°

45°

37°

35°

33°

31°

100Km

Figure 1. Plate tectonic confi guration of the area around the Aegean (modifi ed from Papazachos et al. 1998). SK– Skopje, RZ–

Rhodope Zone, VR– Vardar Zone, TB-WA– Th race Basin-Western Anatolia.

before the Early Miocene (Dinter & Royden 1993;

Dinter 1994; Dinter et al. 1995). Th e Aegean region

has experienced back-arc extension, related to the

Hellenic subduction system, from the latest Oligocene

to the present day (McKenzie 1978; Le Pichon &

Angelier 1979; Meulenkamp et al. 1988), while back-

arc extension in the Aegean area was (apparently)

initiated between 15 and 20 Ma ago (Angelier et al.

1982; Jolivet et al. 1994). Th e extension started to be

modifi ed about 5 Ma ago, aft er the North Anatolian

Fault had started to open the Sea of Marmara pull-

apart basin and crossed the Dardanelles (Armijo et

al. 1999). If so, the formation of sedimentary basins

in the NE Aegean Sea (e.g., Lemnos) and Oligocene

magmatic activity in the Rhodope area may be related

to compression rather than extension. During the

Late Eocene–Early Oligocene, Lemnos was a forearc

basin of the ‘contracted’ type with the outer arc ridge

(that oft en serves as a dam to pond sediments in the

forearc region) as a major contributor of sediments

into the forearc basin (Maravelis & Zelilidis 2010b)

(Figure 2). Th is source, which delivered ultramafi c,

gabbro, basalt, chert and, possibly, some volcaniclastic

detritus of variable grain size into the adjacent forearc

basin (Lemnos), should be located south-southwest

of Lemnos (Central Aegean region?). Th e source area

was probably rugged, with vigorous and erosion,

causing the ophiolitic bedrock to be incised deeply

and rapidly, allowing a signifi cant amount of coarse-

grained material, from a rapidly uplift ing source area,

to be made available for sedimentation (Maravelis &

Zelilidis 2010b).


418

LATE EOCENE-EARLY OLIGOCENE

OUTER ARC RIDGE

(CENTRAL AEGEAN SEA?)

MAGMATIC ARC

(SOUTHERN RHODOPE)

SUBMARINE FAN DEPOSITS

(LEMNOS ISLAND)

N S NW

TUR

KEY

?

AFRICAN PLATE

EURASIAN PLATE

10 km

Figure 2. Schematic diagram illustrating the depositional setting of Lemnos (from Maravelis & Zelilidis 2010b).

Th us, the Palaeogene is characterized by

deposition in a submarine fan system. Deep-water

sediments at the base of the stratigraphic succession

in Lemnos take the form of a conventional sand-

rich submarine fan system made up of monotonous

alternations of sandstone and mudstone. Sediments

consist of very thin- to very thick-bedded sandstones

and conglomerates, interbedded with hemipelagic

mudstones. Sandstones are light brown to light green

displaying both complete and incomplete sets of the

Bouma sequence. Conglomerates are disorganized

or have rare inverse to normal grading, are polimict,

and consist of radiolarian, calcareous, arenaceous,

gneissic schist, or quartzitic cobbles in an arenaceous

cement. Mudstones are brownish and typically lack

internal structure although beds with silt laminae

have been observed (Maravelis et al. 2007).

Th e overlying shelf environment is characterized by a general fi ning upward trend. At its base are sandstones that are interbedded with very thin mudstone beds. Many sandstones appear featureless, although others show grooves and tool marks. Internal structures are dominated by a prominent

parallel lamination. Th e sandstone beds show, generally a single set of ripple cross-laminae at the top. Mudstones commonly contain a high proportion of coal debris. Upwards, this unit grades from a sand-dominant to an almost completely mud-dominant sequence that consists of massive, homogeneous green or green-grey mudstones (Maravelis et al. 2007).

During the Miocene, Lemnos was the site of volcanic activity and magmatic rocks overlie the shelf deposits (Pe-Piper & Piper 2001). Magmatic rocks consisting of both plutonic and volcanic rocks, are principally trachyandesites and dacites, and cover a large part of the studied area. Th e igneous rocks are considered to belong to the one high-K province along the Aegean-Anatolian-Frontier, the Northern one, the ‘Shoshonitic Province’ of Pe-Piper et al. (2009) that includes the islands of Samothrace, Lemnos and Lesvos and runs 200 km into Western Anatolia and the Northern part of Chios and İzmir (Smyrna ). Th ese high-K rocks, mostly of intermediate composition, indicate ensuing calc-alkaline orogenic volcanism, emitted from large volcanic centres. Upwelling of asthenospheric mantle has been invoked to account


419

for their genesis (Pe-Piper et al. 2009). Th e end of

the Miocene is characterized by the deposition of

conglomerates, marls and calcareous sandstones.

Local Pleistocene porous calcareous and locally

oolitic limestones and Holocene alluvial, coastal

deposits and dunes are sparse in Lemnos (Figure 3).

Methodology

Samples were selected for both porosity-permeability

and grain-size investigation with a view to investigate

the entire stratigraphic succession of the studied area

(Figure 4) and the lateral extent of the sedimentary

units (Figure 5). Porosity-Permeability data were

obtained using the ‘Mercury Porosimetry Technique’

as proposed by Ritter & Drake (1945) and Katz &

Th ompson (1986, 1987) on 20 sandstone samples.

Th ese analyses were undertaken at the Institute

of Chemical Engineering and High Temperature

Chemical Processes, Patras, Greece. Most of the

selected samples are turbiditic sandstones, while

one shelf-derived sample was selected for reservoir

analysis (Figure 4).

In order to evaluate the grain-size, 30 sandstone

samples (28 of turbititic origin and 2 of shelf-derived)

were collected, prepared and examined under a

polarizing microscope. Grain-size was determined

using the method of Johnson (1994) and 300 detrital

grains were counted in each sample. Th is number can provide accurate results even in medium-grained sandstones. By means of point counting the standard deviations were also calculated and were determined the grade of sorting of the selected sandstones. Johnson (1994) suggested that standard deviation of 0.45 φ or less characterize very well-sorted sandstones, standard deviations of 0.45 to 0.55 φ well-sorted sandstones, values of 0.55 to 0.70 moderately-sorted, 0.7 to 0.9 φ poorly-sorted while very poorly-sorted sandstones are designated by

standard deviation values greater than 0.9 φ.

Reservoir Characteristics

Sedimentary Patterns

Th e study area largely comprises turbidites that have

been interpreted as parts of a sand-rich submarine fan

in a base of slope to basin fl oor environment, overlain

by shelf deposits. Th e turbidity system is composed

of a ‘basin fl oor’ fan underlying a ‘slope’ fan, and

was constructed under the synchronous interaction

of both progradation and aggradation processes.

Th e ‘basin fl oor’ fan is the more distal and lower,

unchannelized fan and is composed of lobe, lobe-

fringe and fan-fringe deposits. Th e ‘slope’ fan consists

of channel-overbank deposits, demonstrating greater

proximity to the source area (Maravelis et al. 2007).

Bedding in the ‘basin fl oor’ fan is relatively simple,

parallel, and regular, while the lateral bed continuity

is relatively high. ‘Slope’ fans display a complicated

bedding pattern with vertical and lateral random

distribution of channel fi lls, axial erosion, and bed

convergence towards the channel margins. Th e

system presents an overall braided character to the

fan surface with the formation of both sheet-like and

lobate sand bodies, due to the low volume of fi ne-

grained material within this system that prevents the

development of confi ned and stable channels.

Th e CC (sensu Posamentier et al. 1988;

Posamentier & Allen 1999) is not visible in the

submarine fans of the study area; hence the change

in strata stacking patterns from highstand normal

regression to forced regression cannot be mapped.

Th us, the studied sediments suggest deposition

slightly aft er the onset of forced regression, passing

through the two stages of forced regression until

sub-marinefans

shelf

coastal deposits

dunes

alluvium

porous, calcareousand

oolitic limestones

conglomerates,marls and

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Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

volcanicrocks

late

early

late

early

late

early

late

mid

CenozoIc

Figure 3. Generalized chart of the Late Eocene to Holocene

stratigraphy of Lemnos, showing the position of the

studied sediments.


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Table 1. Porosity (%) values and sedimentary facies of 20 selected outcrop samples from Lemnos sandstones (see fi gures 4 and 5 for

their exact location within the sedimentary succession and for sample distribution on Lemnos respectively).

Sample No Porosity (%) Sedimentary facies Sample No Porosity (%) Sedimentary facies

1 2.4 lobe-fringe 11 12 channel-fi ll

2 12.3 lobe-fringe 12 15.4 channel-fi ll


4 24.8 lobe 14 3.0 channel-fi ll


6 12.2 lobe 16 11 channel-fi ll



9 27 channel-fi ll 19 14.8 channel-fi ll

10 8.1 channel-fi ll 20 14 shelf

the onset of lowstand normal regression (Maravelis & Zelilidis 2011). Organic-rich mudstone or shale is commonly interbedded with turbidites and these could serve as both source rocks and seals.

Porosity-Permeability of Outcrop Samples

Porosity is the void space in a rock. It is commonly measured as either a volume percentage or a fraction (expressed as a decimal). In this study the percentage form is used, revealing that the selected sandstones display porosity values that oscillate between 2.4% to 27% (Table 1). Permeability and reservoir quality are a function of how the pore spaces are connected, their type and distribution, and the pore throat sizes. Th e application of the ‘Mercury Porosimetry Technique’on the Palaeogene sandstones of Lemnos indicate that the selected samples display a wide range of permeability values that oscillate between 0.0039 mD and 154 mD (Table 2). Th e application of Levorsen’s (1967) classifi cation suggests that, according to their porosity and permeability values, selected samples are of four types: non-reservoirs, with values 0-5% and <1 mD (samples 1 and 14); low-reservoir quality, with values 5–10% and<1 mD (samples 2, 3, 5, 7, 8, 10, 11, 13 and 16); moderate reservoirs, with values 10–15% and 1–10 mD (samples 6, 9, 17, 18 and 19); good reservoirs, with values 15–20% and 10–100 mD (samples 12 and 15)

and very good quality reservoirs with values 20–25%

and >100 mD (sample 4). Permeability value is a

primary index to defi ne the potential of the rocks

to form oil or gas reservoirs. Th e exact value will

depend upon the nature of the petroleum and the

complexity of the reservoir. Permeability may exceed

10D in the best reservoirs, but in many reservoirs it

may only be tens to hundreds of mD. At the lower

end, gas may be produced from reservoirs of 0.1mD

(samples 2, 3, 6, 9, 13, 16, 17 and 18). Gas reservoirs

can still perform with a mean permeability of as little

as 1 mD (samples 6, 9, 17, 18 and 19), while 10 mD is

oft en accepted as the lower limit for light oil (samples

4, 12 and 15).

Pore-Th roat Structure Probed by Mercury Porosimetry

Heterogeneities from the complex pore-throat

structures of sandstone reservoirs in continental

environments may yet be an important textural

feature, playing a crucial role in the transport

behaviour of reservoir liquids and prediction of

the next-step strategies of petroleum exploration

and exploitation (Chowdhury & Noble 1992;

Sachsenhofer et al. 2006).

Mercury porosimetry is a powerful tool to

eff ectively probe the pore structures at the meso-to

macro-scales (4–400 mm). In mercury porosimetry


423

Table 2. Permeability (mD) values and sedimentary facies of 20 selected outcrop samples from Lemnos sandstones (see fi gures 4 and

5 for their exact location within the sedimentary succession and for sample distribution on Lemnos respectively).

Sample No Permeability (mD) Sedimentary facies Sample No Permeability (mD) Sedimentary facies









9 3.6 channel-fi ll 19 3.3 channel-fi ll

10 0.071 channel-fi ll 20 0.051 shelf

experiments, the volume of mercury penetrating into

or receding from porous samples can be measured

as a function of the applied hydraulic pressure. Th e

obtained mercury intrusion and extrusion curves may

be theoretically interpreted as pore size distributions

in terms of the Washburn equation (R= 2γcosθ/P),

in which the applied hydraulic pressure, p, is related

to the cross-sectional radius, R, of pore-throats

accessible by the pressured mercury, together with

two material-related thermodynamic parameters:

surface tension of mercury, γ, and its contact angle, θ,

with the sample material involved (Washburn 1921;

Leon y Leon 1998).

Typical mercury intrusion curves, i.e. frequency

versus pore throat diameter, are presented in Figure

6. Th ey have similar profi les displaying two pore-size

distributions, suggesting major textural heterogeneity

(except sample 1). Th is feature could aff ect Lemnos

reservoir quality by decreasing permeability. Even

though sandstones display permeability values lower

than their great porosity values (more than 10%),

this feature was not of great importance since their

permeability values are suffi cient to consider these

rocks as both oil and gas reservoirs.

Sandstone Texture and Composition

Th e studied sandstones are light brown to light green

in hand specimen and are commonly interbedded

with brownish mudstone (Maravelis et al. 2007).

Compositionally they are lithic-arenites; point

counting of thin-sections has shown that these

rocks have stable clastic ingredients with the most

common detrital grains being 40–58.4% quartz,

10–23% feldspar and 16.2–35.7% lithic fragments

(Maravelis & Zelilidis 2010b). Aft er quartz, lithic

fragments are the most abundant component in the

Lemnos sandstones. A wide variety of fragments

has been observed in thin sections, including

metamorphic (schist rock fragments), sedimentary

(microcrystalline chert and sandstone rock

fragments) and igneous (felsic plutonic granite and

mafi c volcaniclastic basalt fragments) lithic fragments

Petrofacies analysis suggest that the metamorphic,

sedimentary and plutonic igneous rocks, in a recycled

orogenic environment, were the most important

source rocks for the studied sediments (Maravelis &

Zelilidis 2010b) (Figure 7).

Apart from these principal components,

petrographic modal analysis revealed the presence


424

of a variety of heavy and accessory minerals, most

commonly well-rounded, green/brown rutiles

and tourmalines, while apatite, biotite, muscovite,

chlorite and glauconite grains are the most important

accessory minerals (Maravelis & Zelilidis 2010b).

Lithological features for each sample are

summarized in Tables 3 and 4. Th e studied

sandstones are classifi ed as very fi ne grained to

medium grained and comprise mostly moderately to

very well-sorted lithic-arenites. Th ere is no obvious

relationship between sub-environments and textural

parameters in these sediments, whereas the fi ner the

sand grain-size, the better-sorted the sandstones. In

general, textural parameters such as grain-size and

sorting have eff ects on the porosity and permeability

of reservoir facies. Th e amount of porosity lost will

depend largely on how well-sorted the sand is. In a

poorly-sorted sand, more porosity will be lost than

in well-sorted sand, with the little grains fi lling in

between the bigger ones (Vesic & Clough 1968). Th e

fi ner the sand grain-size, the lower the permeability.

Th e better-sorted sandstones tend to have higher

porosity (Beard & Weyl 1973).

Figure 6. Typical mercury intrusion curves, frequency versus pore throat diameter. TSD– Th roat-Diameter

Distribution, PSD– Pore-size Distribution.


425

Figure 6. Continued.

Discussion

Th e NE Aegean Sea and surrounding areas have been

the focus of hydrocarbon exploration during the

last two decades of the 20th century. Many authors

have described the oil and gas prospectivity of this

tectonically active area, which is now considered to

be a proven hydrocarbon province. Th e Th race Basin,

considered as a fore-arc basin by Görür & Okay

(1996), provides a great example of the petroleum

potential of the NE Aegean (Turgut et al. 1983).

Exploration mainly targeted deep plays in Eocene

sediments, resulting in several discoveries. Currently

there are 14 commercial gas fi elds and 3 oil fi elds.

A recent discovery has been made in shallower

Oligocene sediments (http://www.amityoil.com.au).

Similar hydrocarbon potential studies calculating the

hydrocarbon potential have been performed in NW

Anatolia by Kara Gülbay & Korkmaz (2008).

Given the proven hydrocarbon potential of the

NE Aegean Sea and surrounding areas, this study


426

attempts to add one more argument for further in-

depth exploration by means of reservoir characteristic

evaluation of similar to proven oil and gas fi eld

deposits.

Geochemical and petrographic data suggest that

Lemnos sandstones were deposited in an active

continental margin environment and represent

the infi ll of a fore-arc basin of ‘contacted type’. Th e

studied area, during the Palaeogene, lay between

the active volcanic arc of the Rhodope Zone, and a

structural high, located in the central Aegean Sea. As

the sediments have an outer arc ridge-arc derivation,

an alternative model of possible multiple sources

for the rocks should be considered (Figure 2). Th is

model does not correspond to the studied area and

needs further examination, including the lateral

correlation of these sediments with coeval submarine

fan deposits in adjacent areas (e.g., NW Turkey)

(Maravelis & Zelilidis 2010b).

Th e stratigraphic architecture of the sedimentary

sequences within the studied area, characteristically

exhibits a successive landward and northward



427


migration of the basin depocentre, possibly because

the accretionary structural high bounding the

oceanward fl ank of the basin was forced upward and

landward by continued accretion at the toe of the

margin. Th e studied turbidity system comprises one

outer and one inner fan approximately 300–350 m

thick, and the outer fan deposits represent the greater

part of the exposed sediments. In the relatively thick

(up to 100 m) channelized fan area on Lemnos, only

one sediment infl ux centre is present. Th e infl ux

centre found in the fi eld is a proximal distributary

channel approximately in centre of the fan (Figures 8

& 9) and contains the coarsest sediment in the study

area (Maravelis & Zelilidis 2011).

Th e regional parts of the study area consist mainly

of basin fl oor fan facies (lobe, lobe fringe, and fan

fringe facies), while slope fan facies (channel fi ll and

levee facies) are restricted to the central parts. Th us,

large regions in the SE and NE of the island consist

of lobe deposits, and their laterally equivalent lobe


428

fringe deposits (Figure 8 & 10). Lobe fringe facies are

represented by underlying slope fan facies (Figure 8

& 11). Slope fan facies are restricted to central parts

in both the northern and southern areas (Figure 8 &

12) while the entire stratigraphic succession is shown

in fi gure 13 (Maravelis & Zelilidis 2011).

Generally, ‘slope’ fan facies, regarding their

porosity/permeability values, provide the most

promising sub-environment for further hydrocarbon

research, in contrast to ‘basin fl oor’ fan facies.

Nevertheless, ‘basin fl oor’ fan facies should also be

the subject of further research, since lobe facies the

greatest values of both porosity and permeability

(Sample No 4). Apart from their reservoir

characteristics, Palaeogene sandstones on Lemnos

crop out extensively, making them important deposits

for further hydrocarbon research. Both ‘basin fl oor

fan’ facies, with their great lateral continuity, and

‘slope fan’ facies with their great thickness and

lateral extent could serve as possible locations for

hydrocarbon accumulation.



429

Figure 7. QFL diagram of the selected samples. All the

sandstones fall in the Recycled Orogen provenance

fi eld (from Maravelis & Zelilidis 2010b).

Table 3. Lithological description for sandstone samples (see Figures 4 and 5 for their exact location within the sedimentary succession

and for sample distribution on Lemnos respectively).

sample no mean grain diameter grain-size sample no mean grain diameter grain-size

1 3.74 φ very fi ne grained 16 3.3 φ very fi ne grained

2 3,41 φ very fi ne grained 17 3.39 φ very fi ne grained

3 2.71 φ fi ne grained 18 2.79 φ fi ne grained

4 3.84 φ very fi ne grained 19 1.9 φ medium grained

5 1.96 φ medium grained 20 2.72 φ fi ne grained



8 1.64 φ medium grained 23 3.46 φ very fi ne grained

9 2.87 φ fi ne grained 24 1.86 φ medium grained

10 3.57 φ very fi ne grained 25 1.87 φ medium grained

11 2.9 φ fi ne grained 26 1.88 φ medium grained

12 2.62 φ fi ne grained 27 3,37 φ very fi ne grained

13 1.94 φ medium grained 28 2.69 φ fi ne grained



Conclusions

Based on a comprehensive study of porosity-permeability evolution as well as sandstone textures, the following conclusions can be drawn for the study area that may provide important information about the reservoir quality of these rocks.

• Th ey can be considered as both oil and gas reservoirs.

• Even though the greatest porosity and permeability values were obtained from a sample derived from a ‘basin fl oor’ fan (lobe facies), ‘slope’ fan facies generally display the most effi cient values, making them the most promising sub-environment for further hydrocarbon research.

• Mostly they display two pore-size distributions, suggesting major textural heterogeneity.


430

Table 4. Standard deviation values and grade of sorting for the studied sandstones (see Figures 4 and 5 for their exact location within

the sedimentary succession and for sample distribution on Lemnos, respectively).

sample no sorting standard deviation sample no sorting standard deviation

1 moderate 0.626 φ 16 moderate 0.629 φ

2 very well 0.441 φ 17 poor 0.759 φ

3 well 0.519 φ 18 well 0.454 φ

4 well 0.526 φ 19 moderate 0.546 φ


6 well 0.545 φ 21 well 0.63 φ

7 moderate 0.668 φ 22 well 0.456 φ

8 well 0.43 φ 23 well 0.455 φ

9 very well 0.474 φ 24 moderate 0.68 φ



12 moderate 0.594 φ 27 very well 0.44 φ


14 moderate 0.642 φ 29 well 0.508 φ


• Sorting was very important during

sedimentation (studied samples are generally

well to very well-sorted. Sandstones with

moderate sorting do occur, while poor sorting

was seen in only one sample).

• Selected lithic-arenites are generally very fi ne-

to fi ne-grained, whereas medium-grained

sandstones are extremely rare and mostly seen

on ‘slope’ fan facies.

• Th e fi ner the sand grain-size, the better-sorted

the sandstones.

Summarizing, the above study suggests that the

Palaeogene forearc sedimentary fi ll on Lemnos, NE

Greece should be the subject of further hydrocarbon

research since the porosity-permeability values allow

their consideration as both oil and gas reservoirs.

Th ese rocks consist of generally fi ne-grained and

well-sorted sandstones.

Acknowledgments

Th is research is co-fi nanced by E.U. - European Social

Fund (80%) and the Greek Ministry of Development-

GSRT (20%). EPAN, PENED ‘03ED497. Authors

would like to thank Mehmet Cihat Alçiçek (subject

editor) and the three anonymous reviewers for

constructive criticism that improved the paper.


431

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437

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